A method in which metal nanoparticles are used as a bonding material is receiving attention as a method of mounting an electronic component such as a semiconductor device on a base material made of ceramic or polyimide or the like. The term “metal nanoparticles” refers to metal particles of a size with a mean particle diameter that is less than 50 nm, such as Au, Ag, or Cu (hereunder, referred to as “metal nanoparticles”). Since the surface activity of such metal nanoparticles is high compared to a bulk material as the result of miniaturization, bonding is possible at a low temperature, and a high fusing point that is equivalent to a bulk material is obtained when bonding occurs and the size increases. Consequently, there are expectations regarding the application of methods that use metal nanoparticles as a bonding material to a wide range of products for which a need exists to decrease thermal stress when mounting electronic components and improve a heat resistant temperature after mounting. Further, compared to a conductive paste material that does not use metal nanoparticles, in a conductive paste material that uses metal nanoparticles, since metal is bonded and not merely touching, the resistivity is low and the bonding strength is also enhanced, and thus superior bonding is achieved. A conventional process of mounting an electronic component using such kind of metal nanoparticles as a bonding material is illustrated in FIGS. 16 to 18.
FIG. 16(a), FIG. 17(a), and FIG. 18(a) are plan views that illustrate a substrate during a mounting step. FIG. 16(b), FIG. 17(b), and FIG. 18(b) are cross-sectional views that represent a section along a line GG′ in FIG. 16(a), FIG. 17(a), and FIG. 18(a), respectively.
First, as shown in FIGS. 16(a) and (b), a substrate 1001 is prepared in which conductor wiring 1002 is formed on a base material. Next, as shown in FIGS. 17(a) and (b), a bonding material portion 1013 is formed at component mounting positions on the conductor wiring 1002 using a paste material that includes metal nanoparticles (hereunder, referred to as “metal nanoparticle paste material”).
Next, as shown in FIGS. 18(a) and (b), electrodes 1014 of an electronic component 1003 are positioned facing the bonding material portion 1013, and the electronic component 1003 is mounted on the substrate 1001. Further, by applying energy such as heat to the bonding material portion 1013, the bonding material portion 1013 is bonded and cured to form joints 1015, and thereby bond the electrodes 1014 of the electronic component 1003 and the conductor wiring 1002 of the substrate 1001. As a result, as shown in FIGS. 18(a) and (b), a mounting structure is produced in which the electrodes 1014 of the electronic component 1003, the joints 1015, and the conductor wiring 1002 are connected in that order in a layered structure.
The metal nanoparticle paste material is stabilized by means of a dispersing agent to prevent polymerization of the metal nanoparticles. By applying energy such as heat to the metal nanoparticle paste material, the dispersing agent in the bonding material portion 1013 is decomposed using oxygen, so that metal nanoparticles fuse together and bond to form the joint 1015.
A method has been proposed that uses this kind of metal nanoparticle paste material when mounting an LED as an electronic component (for example, see Japanese Patent Laid-Open No. 2005-136399). According to Japanese Patent Laid-Open No. 2005-136399, a metal nanoparticle paste is cured by applying ultrasonic vibrations thereto. However, when using ultrasonic vibrations, in some cases damage is caused to the electronic components or the bonding is insufficient. Hence, a method has also been proposed in which curing is performed by heating in the same way as described above together with the use of ultrasonic vibrations or instead of using ultrasonic vibrations.